AD6644: Decoupling/grounding recommendations

Document created by analog-archivist Employee on Feb 23, 2016
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We are using an AD6644 ADC. The data sheet does not stated which GND pins
are associated with the
analogue or the digital sections. Please can you tell me.
Furthermore, could you send any PCB layout/Grounding recommendations when
using this device.

 

Each power pin should be de-coupled directly to it's adjacent ground pin using
a 100nF cap and minimum track length. All ground pins should be connected
together physically close to the IC. The combined ground pins node should then
be treated as an analog ground return path and routed back to the system power
supply via a low impedance ground plane.

High frequency digital ground return currents should be kept separate from the
ADC ground return by using separate analog and digital ground planes. If your
system already has separate ground planes, the ADC ground return current (from
all pins marked GND on the AD6644) should be routed via the analog ground
plane.

We did not mark the ground pins analog or digital as many users interpret this
marking as an instruction to connect those pins marked "Digital Ground" to the
system digital ground plane. In a system which has significant high frequency
digital ground return current, this will inject noise into the ADC and
compromise performance.

The eval board provides limited guidance since it contains minimal digital
circuitry.

The final chapter in all our seminar books is dedicated to hardware design
techniques and deals with such issues as grounding, decoupling, parasitic
thermocouples and good PCB design.
http://www.analog.com/support/standard_linear/seminar_material/index.html

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